某量筒的最大刻度是500mL，用它一次最多能测出酒精____kg。

在x轴的原点O和轴上的P点，分别固定同种电荷Q₁和Q₂，已知Q₁<Q₂，OP距离为2d，则场强为零的坐标x区间为（     ）

20世纪50年代中期，***曾指出：“在中华人民共和国宪法范围之内，各种学术思想，正确的、错误的，让他们去说，不去干涉他们。李森科、非李森科，我们也搞不清楚，有那么多的学说，那么多的自然科学学派。就是社会科学，也有这一派、那一派，让他们去谈。在刊物上、报纸上可以说各种意见。”为此，党和政府提出（      ）

深秋，我国北方常有寒潮活动，农民用烟雾笼罩大白菜地，防止大面积冻害，其主要原理是（   ）

完形填空。

    A boy and a girl catch a bird and 1 it in a birdcage (鸟笼）. They like it 2 and 3 it every day. A cat sees the bird and wants 4 her supper. 5 the bird is in the cage. She can't catch it. So she is very 6.

    One day, she opens the door of the cage 7 wants to catch it. But the bird flies away. The cat 8, and she has 9. The two children 10 and　see the bird isn't in. They are very angry and put the cat into the cage.